1. Check your sizing die. Do you have an 8mm expander ball or a bad 338 ball?
2. How many times have the cases been fired? you may be nearing the end of your case life, and have too much neck thickness. Are some of the cases splitting/cracking on the necks when you re-size?

Thanks for the useful comments. I am using redding FL sizing die for my .338 Lapua. The cases have been fired once with no indications of high pressure after firing or cracking etc after sizing. Possibly the expander ball is not correct or I have them adjusted incorrectly? Obviously the inside case measurment needs to be just snug enough to hold the bullet in place for seating purposes but I don't know what that exact measurement is supposed to be. I bought two pilots for my trimmer thinking that maybe one was sized .338 on accident and a new one might be sized slightly smaller to fit inside the case but labeled for a .338 caliber. I also have a different brand of dies on order so I can measure the differences between the two.

What Mike wants you to understand is the neck inside diameter has to be at least as large as the pilot, ideally a thou larger for easy trimming. Sometimes we have to trim fired cases because of the need for some pilot clearance.

The neck ID is determined by the expander ball, not the die itself. A neck that's some 10 thou smaller than the bullet is NOT right. Ideally it will only be some 1 or 2 thou smaller than the bullet. Any higher bullet "tension" only increases the required seating pressure (but not real bullet pull/tension) AND increases bullet run-out, which is harmful to accuracy.